In recent years, as the air pollution by automotive exhaust gas has been becoming a global problem, an electric vehicle that uses electricity as a power source, a hybrid vehicle that runs by combining an engine and a motor, a fuel cell electric vehicle that uses a fuel cell as a power source, and the like have attracted attention, and a high-capacity and high-output battery mounted on these has occupied an industrially important position. Moreover, also in an automobile that draws power only from the engine, a vehicle that mounts thereon a high-voltage battery enabling a variety of electric instruments to be mounted on the vehicle concerned has been put into practical use. A secondary battery such as a lithium-ion secondary battery is considered to be a battery suitable for such a vehicle since the secondary battery exerts high performance in terms of capacity and output, and a variety of developments are progressed therefor.
Basically, the lithium-ion secondary battery has a configuration in which positive and negative electrodes capable of occluding and emitting Li ions are arranged while interposing a separator therebetween, and these are filled with an electrolyte. The separator is made of a porous one having electrical insulating properties, and is used for the purpose of preventing an internal short circuit that occurs by the fact that that the positive electrode and the negative electrode contact each other, and so on.
In the case of performing charge and discharge in such a battery, when the battery discharges, lithium as a negative electrode component is emitted as the Li ions to the electrolyte, and in the positive electrode, the Li ions are occluded from the electrolyte, whereby electric power is generated. Meanwhile, when the battery charges, the Li ions are emitted from the positive electrode to the electrolyte, and the Li ions in the electrolyte are occluded to the negative electrode. At the same time when the Li ions go out of and come into the electrolyte as described above, electrons from a current collector move through a conductive additive, whereby an electrode reaction progresses, and the charge and the discharge are performed.
A secondary battery for use in a power source for an automobile such as the hybrid vehicle is required to have a large output for a fixed time, that is, a high volumetric output density in order to perform power assistance when the engine starts and the vehicle starts and accelerates. Accordingly, for the purpose of further increasing the output of the secondary battery, as disclosed in Japanese Patent Laid-Open Publication No. 2002-151055, a method is used, which is for enhancing electron conductivity and lithium ion diffusibility in an electrode active material layer while increasing an electrode reaction area by reducing an average particle diameter of an electrode active material and thinning a thickness of the electrode active material layer in the electrode.
Moreover, in Japanese Patent Laid-Open Publication No. 2002-151055, for the purpose of further enhancing the output density of the secondary battery, there is disclosed a secondary battery, in which the active material layer is formed into a structure of two layers different in active material particle diameter, the active material particle diameter of the active material layer on the current collector side is set at 0.1 μm or more to less than 5 μm, and the active material particle diameter of the active material layer on the separator side is set at 5 to 20 μm.
As described above, in the conventional secondary battery, the output thereof is increased by using the electrode active material that is so particularly finely processed that the average particle diameter thereof can be 10 μm or less. However, there has been a problem that such fine processing of the electrode active material causes an increase of an electrode volume, bringing a decrease of an obtained volumetric energy density of the secondary battery.
Moreover, in the case where the secondary battery is used as the power source of the vehicle, and the like, the secondary battery is desired to have a high volumetric energy density in order to further extend a running distance, as well as to have a high volumetric output density, in order to exert excellent output performance. In the secondary battery using the active material layer formed into the structure of two layers different in active material particle diameter, which is disclosed in Japanese Patent Laid-Open Publication No. 2002-151055, though the output density thereof can be enhanced, further enhancement of the volumetric energy density is desired in order to further extend the running distance of the vehicle.
As described above, it is a current situation that the secondary battery is still desired to further enhance the volumetric energy density and the volumetric output density. In this connection, it is an object of the present invention to provide a secondary battery electrode capable of realizing a secondary battery excellent in both of the volumetric energy density and the volumetric output density.